THE THORAX. 543 



opening pisses the trachea, (Esopliagus, axillary and carotid arteries, anterior 

 vena cava, and the pneumogastric, great sympathetic, inferior laryngeal, and 

 diaphragmatic nerves. 



Such is the thoracic cavity. Like the abdomen, it is provided with a serous 

 lining, which will now be examined. 



The Pleura. — The serous lining of the thorax comprises two distinct 

 membranes, named the pUurn, forming two sacs placed one against the other in 

 the median plane, and thus constituting a septmn named the mediastmum, which 

 divides the thoracic cavity into two lateral compartments. Each pleura, there- 

 fore, covere one of the external or costal walls of the thorax, and the correspond- 

 ing moiety of the diaphragm ; it is afterwards reflected in the vertical and 

 antero-posterior plane of the cavity, to concur in the fonnation of the media- 

 stinum, whence it is carried over the lung. This arrangement exhibits the 

 pleura in four portions — a costal, diaphragmatic, mediastinal, together represent- 

 ing the parietal layer of the membrane, and a pulmonary or visceral portion. 



The costal pleura is applied to the inner face of the ribs and the internal 

 intercostal muscles. Strengthened on its adherent face, at each intercostal space, 

 by a layer of yellow elastic tissue, this membrane is related, by its free face, to 

 the external surface of the lung, with which it does not, in a normal condition, 

 contract any adhesions. It is continued posteriorly, by the diaphragmatic layer ; 

 in front, above, and below, by the mediastinal pleura. 



The diaphragmatic pleura adheres somewhat loosely to the fleshy portion of 

 the muscle, but the union is more intimate on the aponeurotic portion. This 

 layer is contiguous, by its free face, with the base of the lung ; it is confounded 

 with the mediastinum by the internal part of its periphery. 



The mediastimd pleura is placed, by its adherent face, against that of the 

 opposite side, and in this way produce the middle septum that divides the thoracic 

 cavity into two portions. Several organs are comprised between the two layers 

 of this partition, but most important of all is the heart. In Veterinary Anatomy, 

 that part of the septum in front of this organ is named the anterior mediastinum 

 — the appellation of posterior mediastinum being reserved for the portion situated 

 behind it. These terms have not the same signitication as in human anatomy, 

 though they are retained here to prevent misunderstanding. 



The anteriar mediastinum, thicker than the posterior, but much less extensive, 

 contains, superiorly, the trachea, oesophagus, anterior aorta and its divisions, 

 anterior vena cava, thoracic duct, and the cardiac, pneumogastric recurrent, and 

 diaphragmatic nerves ; it also includes the thymus gland in the foetus and veiy 

 young animal. Tha posterior mediastinum is incomparably narrower below than 

 above, in consequence of the oblique position of the diaphragm. Its inferior 

 part, always deviated to the left, is extremely thin, and perforated by small 

 openings, which give it the appearance of fine lacework. Traversed altogether 

 superiorly by the posterior aorta, vena azygos, and thoracic duct, this mediastinum 

 gives passage, a little lower between its layers, to the oesophagus, oesophageal 

 branches of the pneumogastric nerves, and to the left diaphragmatic nerve. It 

 is these layers of this mediastinum which pass to the lung to constitute the 

 pulmonary pleura, in becoming reflected above and below, in a horizontal line 

 extending from the root of the pulmonary lobe to the anterior face of the 

 diaphragm. 



The pulmonary or visceral pleura — a continuation, as has been said, of the 

 mediastinal pleura — is in contact, by ite free face, with the parietal layer of the 



